The wind turbine market is changing fast nowadays. There is a continuing demand for larger wind turbines being able to generate a higher number of megawatts of electricity, also referred to as multi-megawatt wind turbines. At the same time the requirements for reduction of size and weight of the turbines and their components becomes more and more important.
In wind turbines, typically a wind turbine rotor drives a low speed shaft of a gear transmission unit or gearbox, which transforms torque and speed of the rotor to the required torque and speed of an electrical generator. The increasing demand for multi-megawatt wind turbines puts a challenging pressure on new designs of components, especially of drive trains, for such wind turbines. This is because weight and cost of the wind turbine are to be kept as low as possible or at least within acceptable ranges, while at the same time it has to be made sure that the components can withstand high rotor loads being generated during operation of the wind turbine.
Furthermore, another important issue to take into account is serviceability. Servicing, e.g. maintenance or component replacement of wind turbine drive train components such as a gearbox and/or generator, is in many cases a difficult and expensive activity, especially for offshore wind turbines. Therefore, wind turbine designs should not only be reliable in order to limit servicing activities, but should also be service friendly such that servicing, when necessary, can be performed easily and at low cost.
Hence, requirements of the wind market today are heading towards more compact designs which are still easy to service.
Conventionally known designs are built as a serial connection of a gearbox, a coupling and a generator, the one behind the other. An advantage of such conventional designs is that enough space is provided for servicing and maintenance of the different components. However, a major drawback of these conventional designs is their high length. This significantly increases the size of the nacelle, which makes the nacelle heavier and increases the cost of the nacelle.
U.S. Pat. No. 7,815,536 describes a drive train for a wind turbine. The wind turbine includes a rotor connected to a low speed shaft which in turn is connected to a gearbox. The gearbox, which is a compound planetary gearbox, has a high speed shaft connected to a generator. The gearbox housing is connected to a generator stator housing. The sun pinion shaft is used as the mechanical input to the generator. The drive train includes a bearing interposed between the gearbox and the generator, by which the generator bearings are fully contained within the gearbox, thereby sharing lubrication and cooling of the gearbox system.
The drive train described in U.S. Pat. No. 7,815,536 is of a compact design. This may reduce the required size of the nacelle, which may lead to a lighter nacelle design. However, a drawback of this drive train is that the gearbox and the generator are both supported by the same bearings or bearing arrangement, and that thus the gearbox and the generator are not formed as two separate or independent entities. This makes servicing and maintenance more difficult and time consuming, and hence increases costs of such servicing and maintenance. There is thus a high demand for more compact designs but which are still easy to service.
Furthermore, the gearbox and generator not being formed as independent entities makes testing of the gearbox and generator more complicated as they cannot be tested separately. This is because test benches are in most cases suitable for gearboxes or for generators. When gearbox and generator have to be tested while being connected together, such test benches might require adjustment so as to be able to perform such testing.